3GPP (3rd Generation Partnership Project) has determined in RAN1#53bis Meeting that downlink bandwidth of LTE-Advanced (Long Term Evolution Advanced) system will adopt Carrier Aggregation technologies which may support a system bandwidth larger than 20 MHz. This brings more flexibility to designs of the downlink control channels, while causing new challenges for choosing one of the designs.
Currently, various companies proposed their outstanding optional schemes respectively. NEC Corporation in its contribution (Downlink control structure for carrier aggregation approach in LTE-Advanced system, R1-083491, RAN1#54bis, NEC, October 2008) summarizes these schemes into five categories.
The first kind of scheme is illustrated in FIG. 1. A control channel is provided on each carrier frequency for transmitting corresponding control information on that carrier frequency. The control signaling on each carrier frequency needs to be decoded independently. This scheme is able to be fully compatible with the LTE system and may reuse existing PDCCH (Physical Downlink Control Channel) structure and DCI (Downlink Control Information) format.
The second kind of scheme is illustrated in FIG. 2. The control channel is provided only on one carrier frequency. The control information for allocating PDSCH (Physical Downlink Shared Channel) data to respective carrier frequencies is encoded and mapped to the control channel region separately. This scheme has the advantages that it needs only to perform decoding for the control signaling on one carrier frequency, existing PDCCH structure and DCI (Downlink Control Information) in LTE system may be reused and backward compatibility may be achieved. However, in this scheme, PDCCHs for all users locate on one carrier frequency, whose bandwidth limits the size of PDCCH, and it is probably necessary to introduce new DCI formats with an indication to the carrier frequency identifier.
The third kind of scheme is illustrated in FIG. 3. The control channel is provided on only one carrier frequency. The control information for allocating PDSCH data to respective carrier frequencies is jointly encoded. This scheme has advantage of further removing redundant control information, however this scheme increases DCI formats as well as the number of blind decoding.
The fourth kind of scheme is illustrated in FIG. 4. A PDCCH may span across all system carrier frequencies and allocate PDSCH data to respective carrier frequencies. This scheme makes PDCCH and DCI format to span across all system carrier frequencies and compresses redundant contents of the control information. The bandwidth available for the PDCCH is wide enough for all user equipments. However, this scheme is not backward compatible and the control information on respective carrier frequencies needs to be jointly encoded. New DCI formats need to be defined and the number of blind decoding will be increased.
The fifth kind of scheme is illustrated in FIG. 5, in which PDCCH is detected according to user capability and its configured system bandwidth. PDCCH of the user locates at a system carrier frequency allocated to the user and the control information for assigning PDSCH data to corresponding carrier frequency is separately encoded. In this scheme, the user equipment only needs to detect some system carrier frequencies.
In a recent 3GPP RAN1 No. 57 Meeting in U.S.A., downlink control channel design of the LTE-Advanced system is divided into two categories after many rounds of discussions (Summary of email discussion on bandwidth extension, R1-092219, RAN1 #57, Nokia, May 2009). The first category configures a separate PDCCH to each carrier frequency. The second category allocates a common PDCCH to multiple carrier frequencies allocated to a certain user and the control information is jointly encoded. The first category is further divided into two sub-categories. In the first sub-category, one PDCCH is used to only indicate resource allocation information on a carrier frequency where the PDCCH locates. In the second sub-category, a PDCCH may be used to indicate resource allocation information on a carrier frequency where the PDCCH locates and on other different carrier frequencies. FIG. 6 concretely shows that PDCCH in Option 1a is used to indicate PDSCH allocation information on a carrier frequency where the PDCCH locates and PDCCH on carrier frequency 1 in Option 1b may be used to indicate not only PDSCH allocation information on the carrier frequency (carrier frequency 1) where the PDCCH locates but also PDSCH allocation information on neighboring carrier frequency (carrier frequency 2). Current discussion result is to make Option 1a as a fundamental scheme and to further study Option 1b. 
Above schemes such as Option 1a and Option 1b have their advantages and disadvantages. Option 1a may save carrier frequency indication bits and may be fully compatible with LTE Release 8 structure. Option 1b may flexibly allocate resources. If advantages of these two schemes may be combined, system performance may be further improved and better resource utilization efficiency may be achieved. The present invention proposes a corresponding solution taking into account above issues.